Passive optical network (PON) is a promising approach to meet the ever-increasing bandwidth demand from enterprises and households. PON can be based on different architectures, including but not limited to, Ethernet PON, Gigabit PON, wavelength division multiplexed (WDM) PON. Among these architectures, the WDM-PON is considered as a favourable broadband access solution since dedicated wavelengths are allocated to establish an ultra-wideband bi-directional link between the central office (CO) and each customer. Furthermore, the WDM-PON is cost-effective in the sense that the long feeder fiber used within the network is shared by a large number of customers, whilst offering additional features such as channel independence and per-customer based flexible upgrade. In this type of PON, a cost effective light source, particularly at the optical network unit (ONU) side, is a key component for the practical implementation of the network.
A low cost light source, particularly the uplink light source at ONUs, is the key element for the practical implementation of WDM-PONs. Light sources including spectrum-sliced light-emitting diodes (LEDs), spectrum-sliced free running Fabry-Perot laser diodes (FPLDs) and injection locked FPLDs using spectrum sliced amplified spontaneous emission (ASE) noise and a system exploiting the remodulation of downstream signals received at the ONUs have been considered for the implementation of cost-effective WDM-PONs. Although most of these schemes eliminate the need for wavelength-specific optical transmitters at the customer premises, each scheme has its own drawbacks. The scheme using the LEDs suffers from low power budget while the scheme comprising spectrum slicing of a free-running FPLD suffers from strong intensity noise. The injection locking of FPLDs using spectrum sliced ASE requires high ASE power for high bit rate operation while the re-modulation scheme needs further development to suppress the crosstalk from the residual downlink data and also to alleviate the dependence of the polarization state of the downlink data.
The concept of using amplified spontaneous emission (ASE) directly as uplink light source has also been proposed for the WDM transmission system. However, as a result of the noise characteristics of the ASE light sources, the transmission performances of the system are limited in terms of bitrate, distance and receiver sensitivity, amongst others.
A recent US patent to Jea-Hyuck Lee et. al., Publication No. US 2004/0175177 A1, proposed using self-seeded reflective semiconductor optical amplifiers (RSOAs) as optical network unit (ONU) light sources. In this scheme, as shown in FIG. 1, the ONU light source or transmitter 100 consists of a reflective semiconductor optical amplifier (RSOA) 102 and a reflection-type optical fiber Bragg grating (FBG) 104 located at a predetermined distance from the semiconductor optical amplifier 102 along the fiber 106. During operation, the optical transmitter 100 transmits an output light 108 of a preset wavelength resonating between the RSOA 102 and the reflection-type optical FBG 104. This occurs as a laser cavity 110 is formed between the RSOA 102 and the FBG 104, whereby only the light having a wavelength within the reflective spectrum of the FBG 104 is oscillated to achieve single mode operation. As a result of the broad spectrum of the RSOA 102, the operation wavelength of the ONU light transmitter 100 can be determined by the resonant wavelength of the FBG 104. The wavelength of the output light 108 can be tunable by using different FBGs with different resonant wavelengths. Alternatively, the resonant wavelength of a single FBG can be tuned to produce output light of different wavelengths via changes in temperature and/or pressure. In both scenarios, the stability of the FBG(s) will be a critical challenge for practical implementation.
In order to improve the stability and better arrange the wavelength of the WDM-PON, a modified network architecture 200 has been proposed by E. Wong et. al., Electronics Letters, Vol. 42, No. 5, 2 Mar. 2006, as shown in FIG. 2. The architecture 200 comprises a remote node (RN) 201 consisting of a cyclic arrayed waveguide grating (AWG) 202, an optical coupler 204, an optical circulator 206 with three ports 208, 210, 212 and a bandpass filter (BPF) 214. The architecture 200 also comprises an optical line terminal (OLT) 216, consisting of an arrayed waveguide grating (AWG) 218 and a number of transmitter/receiver modules, e.g. 220, 222, whereby each transmitter/receiver module, e.g. 220, 222, comprises a transmitter 224, an upstream receiver 226 and a wavelength division multiplexed (WDM) filter 228. The OLT 216 is connected to the RN 201 via a feeder fiber 230. The architecture further comprises a number of optical network units (ONUs), e.g. 232, 234, whereby each ONU, e.g. 232, 234, comprises a reflective semiconductor optical amplifier (RSOA) 236 receiving upstream data 238 as the input, a wavelength division multiplexed (WDM) filter 240 and a downstream receiver 242. Each ONU, e.g. 232, 234, is connected to the RN 201 via a distribution fiber, e.g. 244, 246.
Within the network architecture 200, the downstream signals (λ1-D, λN-D) and the upstream signals (λ1-U, λN-U) are separated into wavebands that are spaced at a multiple of the free spectral range (FSR) of the AWGs, e.g. 202, 218. These wavebands are combined and separated by WDM filters, e.g. 228, 240, at the optical line terminal (OLT) 216 and the optical network units (ONUs), e.g. 232, 234, respectively. At each ONU, e.g. 232, 234, an RSOA, e.g. 236 emits a broadband amplified spontaneous emission (ASE) spectrum which is spectrally sliced by the AWG, e.g. 202, 218, in the upstream direction, and the BPF 214 ensures that only one spectrally sliced light per output port is passed through and reflected back to each RSOA, e.g. 236, within the ONUs, e.g. 232, 234, for self-seeding. As a result of the double passed insertion loss from the AWG 202 and the coupler 204 in the RN 201, an optical amplifier (not shown) is incorporated into the RN 201 to provide gain for the feedback signals. However, incorporating active components at the RN 201 is not desirable for the practical network implementation and should be avoided.
A need therefore exists to provide a remote node for a WDM PON, a WDM PON and a method of amplifying self-seeding portions of respective uplink optical signals in a WDM PON that seek to address at least one of the above-mentioned problems.